The present invention relates to a novel artificial ear and auditory canal system, and a means of manufacture of the same.
The invention has particular application in the field of binaural, three-dimensional sound recording and associated techniques, and also in the fields of noise measurement and hearing prostheses development.
Artificial head recording systems are now well known (see for example U.S. Pat. No. 1,855,149) A typical artificial head system comprises a pair of microphones mounted on to the sides of an artificial head assembly where the auditory canal would be, inset into a pair of artificial pinnae (the visible ear flaps). A recording made with an artificial head incorporates many of the 3D sound xe2x80x9ccuesxe2x80x9d which our brains use to interpret the positions of sound sources in 3D space, and so such recordings provide quite dramatic 3D effects when auditioned over headphones. More recently, it has become possible to make acoustic measurements on artificial heads (the measurement of Head-Response Transfer Functionsxe2x80x94HRTFs), and synthesise the effects of the head and ears electronically, using digital signal-processing. However, although these effects are initially perceived to be quite dramatic, especially when heard for the first time, several major deficiencies in present-day artificial heads become apparent when they are tested more rigorously.
The two prime deficiencies are (a) poor xe2x80x9cheightxe2x80x9d effects, and (b) poor front-back discrimination. For example, in respect of (a), this means that when a recording is made of a sound-source moving over the top of the head (from, say, a position close to the left ear, over the head to a position close to the right ear), then the sound-source appears to move directly through the head, rather than over the top. In respect of (b), if a recording were made of a sound-source moving around the artificial head in the horizontal plane in a circle of constant distance (say 1 meter), then the recorded source would appear to move back and forth in arcs from the left ear to the right, always in front of the listener and never behind. These spatial inaccuracies are often overlooked or ignored for recording purposes, where most real-life sound-sources are in front of the artificial-head/listener, and not in these more extreme positions. Nevertheless, the poor spatial accuracy of presently available artificial heads prevents the synthesis of an adequate 360xc2x0 sound-field, such as is required for computer games applications, immersive virtual reality, simulators and the like.
Many researchers have been puzzled over why their artificial head systems are inadequate in the above respects. Some have turned to making measurement on real head-ear systems, by embedding miniature microphones in the pinnae or auditory canals of experimental volunteers. Others have resorted to building their own artificial head systems, attempting to improve on the products of commercial manufacturers, and, in some cases, have taken molding from the ears of volunteers for replicating and using. In one extreme example, U.S. Pat. No. 4,680,856 (Zuccarelli) attempted to replicate or simulate the entire anatomy of the skull, including the bones, double-twisted oval auditory canals, Eustachian tubes, teeth and skin, in order to copy reality as closely as possible. Zuccarelli even stated that a wig was necessary in order to provide good front-back discrimination! Clearly, this latter approach is totally unsuitable for a manufactured product in terms of expense and operational factors (weight, bulk and appearance). In addition, this approach does not allow for the creation of a system with adequate Left-Right matching, because very small L-R differences, introduced during manufacture, in the size, shape or position of any of the acoustic cavities in the structure create significant differences in the overall properties and HRTFs.
The first demonstration of a stereophonic effect is believed to have taken place in Paris in the 1890s, when multiple microphones situated in an array across the front of a stage were each connected to individual ear pieces in an adjacent room, and listeners found that the use of adjacent pairs of ear pieces (and hence microphones) provided very realistic sound reproduction with spatial properties. The first explicit report of a dummy-head type of sound reproduction method appears in U.S. Pat. No. 1,855,149, dated 1927 in which the purpose was to record sounds in such a way that the natural, head-related time-of-arrival and amplitude differences between L and R signals were convolved acoustically on to the sounds, and then replay was achieved using either earphone reproducers or equidistant loudspeakers, placed directly to the left and right of the listener, such that xe2x80x9cthe virtual sound origins were securedxe2x80x9d. British Patent No.394325 (Blumlein) filed in 1931 relates to conventional, present-day stereo in which the use of two or more microphones and appropriate elements in the transmission circuit were used to provide directional-dependent loudness of the loudspeakers, together with means to cut discs and thus record the signals. Stereo sound recording and reproduction was not commercially exploited widely until the 1950s.
At the present time, conventional stereo is largely Blumlein""s amplitude-based stereo, in which a number of individual, monophonic recordings are effectively xe2x80x9cplacedxe2x80x9d spatially in the sound-stage between the listener""s loudspeakers by virtue of their L-R loudness differences. This is achieved by xe2x80x9cpan-pottingxe2x80x9d. It is possible to add artificial reverberation and other effects to enhance the spatial aspects (room acoustics, and distance) of these recordings.
When live recordings are being made, it is common to use stereo microphone pairs, placed so as to be either (a) coincident, or (b) spaced-apart by about one head-width, or thereabouts. This latter goes part-way to the reproduction of a natural acoustic image of a performance, but there have been several periods since the 1950s when the use of the dummy-head recording method for producing binaural signals has been experimented with for improving the quality of the stereo image.
Historically, the term xe2x80x9cstereophonicxe2x80x9d was coined in the 1950s to apply to sound reproduction over two or more transmission channels. In the 1970s, there was a resurgence of interest in recording using dummy-head microphone techniques, and the expression xe2x80x9cbinauralxe2x80x9d was coined exclusively for recordings made by such means. More recently, the term xe2x80x9cbinauralxe2x80x9d has also been used for electronic equivalents, where the acoustic processing effects of the human head and external ear are synthesised.
Dummy-head (binaural) recording systems comprise an artificial, life-size head and sometimes torso, in which a pair of high-quality microphones are mounted in the ear auditory canal positions. The external ear parts are reproduced according to mean human dimensions, and manufactured from silicon rubber or similar material, such that the sounds which the microphones record have been modified acoustically by the dummy head and ears so as to possess all of the natural sound localisation cues used by the brain.
Following on from the development of somewhat crude and simple artificial heads for binaural sound recording in the 1930s and 1940s, acousticians became aware that these head structures were ideal platforms for testing and evaluating hearing aids and other devices, such as hearing defenders (ear-plugs). Consequently, a more academic interest was taken in the development of artificial heads, with more care taken in their construction and engineering. For example, the papers by Torick (An electronic dummy for accoustical testing E. L. Torick et al., J. Audio Eng. Soc., October 1988, 16, (4), pp. 397-402) and Burkhardt and Sachs (Anthropometric manikin for acoustic research M D Burkhardt and R M Sachs, J. Acoust. Soc. Am., July 1975, 58, (1), pp. 214-222) are two excellent papers to study for more information about artificial heads. It soon became clear that, although the simple, earliest head structures were adequate for binaural recording, they were poor representations of the human anatomy. The prime reason is that the early recording heads were fitted with microphones in which the microphone grid was mounted flush with the concha valley floor (see FIG. 1 for ear terminology), and not at the end of a simulated auditory canal. Although this is not a problem for sound recording situations, it is clearly not suitable for the development of in-ear hearing aids, where the actual presence and acoustic impedance of the auditory canal itself becomes an important feature. In order to remedy this omission, Professor Zwislocky, of Syracuse University, devised an acoustic coupler to mimic the properties of the auditory canal. This was described in several internal University reports, and was later developed commercially for use in the KEMAR manikin by Knowles Electronics, (U.S. Pat. No. 5,033,086) who improved on the original structure from the manufacturing point of view. The Zwislocki coupler is a stainless-steel, cube-like structure, measuring 21.5xc3x9721.5xc3x9715 mm, featuring an entrance port on one face, for coupling to an artificial ear, and a 12 mm microphone port on the opposite face. On each of the remaining four faces, there is coupled a small, tuned acoustic circuit side-branch. Each side-branch has a particular specific inertance, resistance and compliance, such that the overall impedance versus frequency characteristics of the coupler match those of the average adult human, with great accuracy, up to about 8 kHz. Beyond this, it was supposed that the reflective surface of the microphone diaphragm becomes too dissimilar to that of the eardrum to accommodate.
In terms of acoustic research, this form of ear coupler, together with similar products made by different manufacturers, became adopted for applications where very high accuracy of auditory canal simulation was necessary. However, for audio recording, the auditory canal presents a severe practical problem, in that the primary quarter-wave resonance of the auditory canal simulator creates a very substantial boostxe2x80x94often 10 to 15 dBxe2x80x94at around 3.9 kHz, and this adds to the equally substantial resonance of the concha cavity at about 2.8 kHz. The consequence is that there is a major 25 to 30 dB resonant peak at around 3 kHz which must be compensated, or else the recordings are tonally very incorrect. Correction of such a gross anomaly is possible. It is difficult to achieve using analogue methods, but is feasible using digital filtering. However, even when this is accomplished, there is still a signal-to-noise penalty to pay, because the resonant boost has effectively pushed the non-resonant regions of the response by 30 dB towards the noise floor of the system. Additionally, the use of 12 mm microphones mandates the use of non-studio type microphones, with poorer noise performance. For these reasons, non-auditory canal based head systems are still preferred for studio recordings, where the best possible signal-to-noise ratios are demanded. Research by Shaw and Teranshi (Paper entitled xe2x80x9cSound Pressure Generated In An External-Ear Replica and Real Human Ears By a Nearby Point Sourcexe2x80x9d by E A G Shaw and R Teranshi, J. Acoust. Soc. Am., 1968, 44, (1), pp. 240-249), indicated that the sound pressure levels (SPLs) scale linearly from the auditory-canal entrance to the eardrum, and so the use of artificial heads without auditory canal simulators has been claimed to be valid. However, this result must be viewed with great caution, because of their experimental methods, since introducing even the smallest measurement transducer into either the pinna or auditory canal affects the overall acoustic properties of the ear in a substantial way.
There are several types of artificial heads available commercially at the present time. The following four, described below, are the most widely used types, although we have heard of several other Japanese and American types from smaller manufacturers. The main features are noted below.
A known artificial head (BandK type 4100) manufactured by Bruel and Kjaer features an artificial head mounted on to a torso simulator, fitted with a sound dampening fabric, which fits over the neck of the manikin. The head is in the form of a hollow xe2x80x9cshellxe2x80x9d, with the microphones mounted directly on to metal plates on the sides of the shell assembly. The neck can be adjusted so that it tilts forwards, to an angle of 17 degrees. The pinna simulators are silicone rubber types, dimensioned to EC 959 and CCITT P.58, except for the ear-canal extensions, with BandK 4165 microphones mounted in the concha cavity. Overall weight is 7.9 kg.
Another known artificial head, the Ku 100 is the successor of the well-known Ku80 and Ku81 series heads which have been manufactured by Georg Neumann GmbH and used since the late 1970s. The Ku80 was improved and renamed Ku81 in 1981, and there have been several variants using xe2x80x9cixe2x80x9d affixes claiming improved loudspeaker compatibility (this might relate to changes in the EQ filters). The head is a solid, rubber-filled element, which can be spilt front-back to access the microphones and battery compartment. The head is fitted with artificial auditory canal-type microphone couplers, and uses Neumann 21 mm, KM100 series miniature condenser microphones, with in-built FET preamps. The head is fitted with electronic equalisation, probably analogue filters, which is battery driven and is located in the head itself The head is suitable for hanging or tripod mounting, and does not have shoulders. It weighs 2.7 kg, and is matt black.
Another well known artificial head, the Aachen (Head Acoustics) system 15 manufactured by Head Acoustics GmbH (see U.S. Pat. No. 4,631,962) is different to other artificial heads in that it is based on a much-simplified structure, which the inventor claims is representative of the important features of human hearing. The ear shapes and head dimensions conform to a set of equations which simplify the construction of the head. It was developed initially for noise measurement in the automotive industry. The head is suitable for tripod mounting, and has shoulders which can be attached, if required. It weighs 7 kg, and is matt black. An equalisation unit is usually supplied with the head.
A further well known artificial head system is the KEMAR manufactured by Knowles Electronics Inc., [Knowles Electronics Manikin for Acoustic Research.] This manikin system was developed in the 1970s, and has been widely used for the research and development of hearing aids. The system is available in modular form, including an optional torso. The head is hollow, splitting around the upper skull periphery, and the inner surfaces have been coated with lead-filled epoxy in order to dampen any resonances reduce the transmission of sound through the shell itself. 12 mm BandK microphones are fitted to the shell using Zwislocki couplers, and the coupler inlets are connected directly to openings in the silicon rubber pinnae. The pinna rubber is a mixture of two different types in order to simulate as closely as possible the mechanical properties of the human ear. Several different neck units are available, with differing heights. Various ear types are available, too, for different applications.
None of the aforementioned commercial heads give adequate xe2x80x9cheightxe2x80x9d cues, and they also have poor front-back discrimination, due to the relative inefficiency of the artificial ears that have been used in the past.
Some researchers have replicated ears by taking molding from either real ears or sculpted copies of real ears. However, this is not satisfactory for the following reasons.
(a) The Left to Right matching is very poor, and cannot be corrected or adjusted.
(b) Molding errors are present, which introduce shrinkage and distortion.
(c) There is no control over the dimensions, and so particular values cannot be specified.
(d) The mating arrangements between the ear unit and the auditory canal or microphone mount are not well-defined. We have discovered that the mating arrangements and the auditory canal or microphone mount are a very critical feature.
It is very difficult to mold artificial ears accurately because of shrinkage of the molded parts. Furthermore it is difficult to use a machine to manufacture a three-dimensional structure such as an ear because of the deep undercuts. It could be achieved, perhaps, by making several 3D xe2x80x9cblocksxe2x80x9d, and then assembling them, but this would be difficult to arrange and would require interlocking alignment lugs in three-dimensional format.
There are many claims in the literature which we have discovered to be incorrect. For example, it is common to claim that the type of materials which are used for the pinnae, skin and other features are important and that artificial ears must be made of materials, such as latex or rubber that have a similar texture or feel as human ears. We have found by experiment and measurement that the material from which the pinna is made is relatively unimportant acoustically, and that the simulation of skin is unnecessary. Duda R O xe2x80x98Modeling Head Related Transfer Functionsxe2x80x99 Proceedings Of The Asilomar Conference, Pacific Grove, Nov., 1-3. 1993, Vol2, Nov. 1, 1993, Institute Of Electrical And Electronics Engineers, pages 996-1000 XP000438445, discloses that Head Related Transfer Functions (HRTFs) characterise the transformation of a sound source to the sounds reaching the eardrums and are central to binaural hearing. Because they are the result of wave propagation and diffraction, they can only be approximated by finitely parameterised filters. The functional dependence of the HRTF on aximuth and elevation is described in this paper, and various artificial head models are described. Many of the described models including that of U.S. Pat. No. 4,631,962 (Genuit), do not replicate the geometry of the human pinna with sufficient precision to produce precise HRTFS. Therefore it is difficult even with with finitely parameterised filters to produce an acceptable HRTF.
The prior art suggests that hard materials are unsuitable for the fabrication of artificial ears for acoustic measurements because their properties are very dissimilar to those of skin. However, we have discovered by comparison of HRTF measurements that, on the contrary, the choice of materials is not significant. Indeed we prefer to use hard materials because of their constancy of physical dimensions (rubber ears can sag and become twisted, thus distorting the shapes and dimensions of their acoustic cavities, and hence significantly changing the associated HRTFs).
An object of the present invention is to provide an accurately dimensioned artificial pinna and auditory canal which provides improved cues as to the height of sources of sound and improved front-back discrimination, utilising materials which conventionally would not normally be considered appropriate for artificial pinnae and which can be manufactured in a controlled, reproducible way, preferably by computer control.
There are known methods of constructing three dimensional articles by building up the article from laminations. Examples of such are to be found in International Patent Applications WO91/12957 and WO87/07538, European Patent Applications 0633129 A1, and 0667227A2, U.S. Pat. No. 5,031,483 and British Patent Application 2,297,516A.
In particular U.S. Pat. No. 5031.483 discloses a technique for making molds by stacking a plurality of sheets, each of which has a shape machined out it. The sheets are stacked to form the finished article.
To an expert in designing artificial pinnae it would not normally be considered appropriate, or desirable, to reconstruct a replica human pinna using a laminated construction because of the creation of multifaceted or stepped edges. One""s initial impression is that such steps or inconsistencies formed at each interface of the laminae would detract from the overall acoustic performance of the artificial ear. On the contrary, we have found that it is possible to xe2x80x98adjustxe2x80x99 the profiles of the laminae (without necessarily eliminating stepped changes from one laminae to the next) and still optimise the overall acoustic performance of the artificial ear.
A further object of the present invention is to provide a means of providing adequate directional information suitable for recording and for providing appropriate data for 3D-sound synthesis. According to one aspect of the present invention there is provided a method of manufacturing a laminated artificial pinna comprising the steps of:
(a) forming a three dimensional model of a human pinna in a first material,
(b) encapsulating said model in a molding material,
(c) machining away the encapsulated model to reveal a cross sectional shape of the model.
(d) making an image of the cross sectional shape revealed by step (c),
(e) repeating step (c) incrementally to reveal cross sectional shapes of the model in spaced parallel planes and repeating step (d),
(f) providing a plurality of blank self supporting sheets of material of a thickness corresponding to the distance between said spaced parallel planes, and using the image produced by step (d) to produce a replica of the cross sectional shape of the model pinna supported from each sheet of material by bridging supports.
(g) repeating step (f) for each cross-sectional shape revealed by step (c), and
(h) assembling and gluing together a stack of said sheets to define a laminated replica of said model.
Preferably step (d) comprises the step of deriving from said image, data for controlling the direction of movement of a cutting tool, and step (f) comprises machining each sheet of material with a cutting tool programmed to move under control of the data derived by step (d).
Preferably step (f) comprises the step of using the image produced by step (d) to produce a mask corresponding to said image, and step (f) comprises the step of removing unmasked material.
The sheets of material may be photosensitive and the unmasked material is removed by exposing the masked sheets to light and a developer.
Preferably an artificial auditory canal is attached to the laminated replica of said model.
The model may be made of a rigid plastics material, and the molding material is a rigid plastics material of a different color to that of the model. The image may be derived by electronically scanning a cross section of the encapsulated model, or derived by photocopying a cross section of the encapsulated model.
Preferably the image is converted to a digitised electronic image. The electronic image may be used to derive a binary computer control code for controlling the direction of movement of a C.N.C. machine cutting tool.
According to a further aspect of the invention there is provided a laminated artificial pinna, constructed in accordance with the latter mentioned method.
Preferably the artificial pinna has a laminated artificial pinna according to claim 12 characterised in that the artificial pinna how a concha, fossa and auditory canal and the auditory canal is constructed and arranged relative to the concha, so that the distance ((A) of FIG. 7) from the center of the entrance of the auditory canal 23 to the rear wall of the concha 12 lies within the range of 15 mm to 20 mm, the distance ((B) of FIG. 8) from the center of the entrance of the auditory canal to the concha floor lies within the range of 9 mm to 15 mm, and the alignment of the turning point ((C) of FIG. 9) with the center of the entrance of the auditory canal is substantially horizontal.
In a preferred embodiment an artificial pinna according to claim 14 herein the bore 27 of our auditory canal 23 comprises a right circular cylindrical bore 27 having a radious and a length ((a) of FIG. 13), measured from an open end of the bore 27 along a central axis of the bore 27 to the plane 29 of the pressure sensitive face 34 of the microphone 33 which is such as to define a resonant cavity having a fundamental resonance of 3.9 KHz.
The bore may be dimensioned so that the dimension of the sum of the length ((a) of FIG. 13) and the radius of the bore equals 22 mm. For example, the diameter of the bore is 7 mm, the angle of the plane of the pressure sensitive face of the microphone is 45xc2x0 to the longditudial axis of the bore, and the length of the bore is 18.5 mm.
Preferably the distance from the central axis of the bore of the auditory canal to the rear wall of the concha is 16.6 mm (average), and the distance from the canal axis to the floor of the concha is 11.3 mm (average).
According to a further aspect of the present invention there is provided a method of recording sound using artificial ears having pinna manufactured according to the method claimed of claim 1, wherein sound waves received by the artificial ears are converted to an electrical signal and are processed by a signal processor having signal filters, the head related transfer functions of which are derived from signal processing algorithms based on measurements corresponding to the measurements of the artificial pinna and auditory canals of the artificial ears which are used to make the recording.